Subsea completion system and method of using same

ABSTRACT

The present invention is directed to various embodiments of a subsea production system. In one illustrative embodiment, a subsea completion system includes a tubing string, a Christmas tree defining a bottom opening adapted to have the tubing string inserted therein, and means for latching the tubing string to prevent movement relative to the Christmas tree. A method includes coupling a tubing string with a Christmas tree above water and landing the Christmas tree on a subsea wellhead.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/517,576, filed Nov. 5, 2003, which is hereby incorporated byreference in its entirety for all purposes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a completion system for oil and gas wells.More specifically, the present invention relates to a subsea completionsystem in which a tubing string is connected directly to a Christmastree. In one embodiment, the tubing string and Christmas tree areinstalled simultaneously.

2. Description of the Related Art

A typical subsea well comprises a high pressure wellhead housing whichsupports one or more casing hangers located at the upper ends of stringsof casing that extend into the well. The system further comprises atubing hanger that supports a string of production tubing through whichthe oil and/or gas products will eventually be produced. Such a systemfurther comprises a production tree or Christmas tree, e.g., ahorizontal or vertical Christmas tree that contains one or moreproduction bores and a plurality of actuatable valves to control theflow of fluids through the production tree.

Conventionally, wells in oil and gas fields are built up by establishinga wellhead housing and, with a drilling blow out preventer stackinstalled, drilling down to produce the well hole while successivelyinstalling concentric casing strings, which are cemented at the lowerends and sealed with mechanical seal assemblies at their upper ends. Inorder to convert the cased well for production, a tubing string is runin through the blow-out preventer and a hanger at its upper end landedin the wellhead. Conventionally, the tubing hanger is latched into awellhead, into a casing hanger inside the wellhead, into an adapterinside the wellhead, or into a tubing head that has been previouslylatched to the wellhead. Thereafter the drilling blow-out preventerstack is removed and replaced by a Christmas tree having one or moreproduction bores containing actuatable valves and extending verticallyto respective lateral production fluid outlet ports in the wall of theChristmas tree.

Such an arrangement introduces many problems that have previously beenaccepted as inevitable. For example, the fluid connection between theChristmas tree and the tubing string has conventionally been made underwater, requiring plugs or barriers to be used to contain well pressure.Such plugs or barriers are conventionally removed via the annuluspassageway, requiring wireline access to the annulus. To providewireline access to the annulus, the annulus valves of the Christmas treeare oriented vertically, resulting in a significantly tall Christmastree.

Connecting the various components of the completion underwater alsopresents other problems. For example, it may be difficult to create aproper seal between the Christmas tree and the tubing string andcorrecting a leak, if one exists, may be difficult. Further, downholehydraulic and electrical service and control lines are connected subsea,which also may suffer from leaks.

Moreover, each individual component that is installed subsea generallyrequires a remote installation and retrieval tool. Such tools may bevery expensive and must be maintained in proper working order, furtherincreasing cost.

The present invention is directed to overcoming, or at least reducing,the effects of one or more of the problems set forth above.

SUMMARY OF THE INVENTION

The following presents a simplified summary of the invention in order toprovide a basic understanding of some aspects of the invention. Thissummary is not an exhaustive overview of the invention. It is notintended to identify key or critical elements of the invention or todelineate the scope of the invention. Its sole purpose is to presentsome concepts in a simplified form as a prelude to the more detaileddescription that is discussed later.

In one aspect of the present invention, a subsea completion systemincludes a tubing string, a Christmas tree defining a bottom openingadapted to have the tubing string inserted therein, and means forlatching the tubing string to prevent movement relative to the Christmastree.

In another aspect of the present invention, a system adapted to becoupled to a subsea wellhead includes a tubing hanger adapted to bepositioned in the wellhead, the tubing hanger defining a flow openingextending therethrough; and a Christmas tree operatively coupled to thetubing hanger above water.

In yet another aspect of the present invention, a method includescoupling a tubing string with a Christmas tree above water and landingthe Christmas tree on a subsea wellhead.

In another aspect of the present invention, a method includes coupling atubing string with a tubing hanger above water, landing the tubinghanger on a subsea wellhead, and landing a Christmas tree on the subseawellhead.

In yet another aspect of the present invention, a subsea Christmas treedefines a lower bore adapted to mate with a tubing string, such that thetubing string is latchable in the Christmas tree.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be understood by reference to the followingdescription taken in conjunction with the accompanying drawings, inwhich the leftmost significant digit(s) in the reference numeralsdenote(s) the first figure in which the respective reference numeralsappear, and in which:

FIG. 1 is a cross-sectional, side view of an illustrative embodiment ofa subsea Christmas tree according to the present invention;

FIG. 2 is a cross-sectional, side view of an illustrative embodiment ofa tubing hanger according to the present invention;

FIG. 3 is a cross-sectional, side view of the tubing hanger of FIG. 2and a tubing string suspended from the subsea Christmas tree of FIG. 1;

FIG. 4A is a cross-sectional, side view of a Christmas tree employing afirst alternative latching mechanism according to the present invention;

FIG. 4B is an enlarged view of a portion of the Christmas tree of FIG.4A;

FIG. 5 is a cross-sectional, side view of a Christmas tree employing asecond alternative latching mechanism according to the presentinvention;

FIG. 6 is an enlarged, cross-sectional, side view of a portion of thelatching mechanism of FIG. 5;

FIG. 7 is a cross-sectional, side view of the Christmas tree/tubinghanger assembly of FIG. 3 installed on a subsea wellhead;

FIG. 8A-FIG. 8K are cross-sectional, side views depicting anillustrative installation sequence of the subsea Christmas tree of FIG.1 and the tubing hanger of FIG. 2;

FIG. 9A-FIG. 9D are cross-sectional, side views depicting an alternativeillustrative installation sequence of the subsea completions systemaccording to the present invention;

FIG. 10 is a cross-sectional, side view of an alternative embodiment ofa completion system according to the present invention;

FIG. 11 is a cross-sectional, side view of an alternative embodiment ofa completion system including a tubing head according to the presentinvention illustrating the tubing hanger of FIG. 2 latched in a tubinghead; and

FIG. 12 is a cross-sectional, side view of an alternative embodiment ofa completion system according to the present invention including atubing head comprising a latching means.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof have been shown by wayof example in the drawings and are herein described in detail. It shouldbe understood, however, that the description herein of specificembodiments is not intended to limit the invention to the particularforms disclosed, but on the contrary, the intention is to cover allmodifications, equivalents, and alternatives falling within the spiritand scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Illustrative embodiments of the invention are described below. In theinterest of clarity, not all features of an actual implementation aredescribed in this specification. It will of course be appreciated thatin the development of any such actual embodiment, numerousimplementation-specific decisions must be made to achieve thedeveloper's specific goals, such as compliance with system-related andbusiness-related constraints, which will vary from one implementation toanother. Moreover, it will be appreciated that such a development effortmight be complex and time-consuming but would nevertheless be a routineundertaking for those of ordinary skill in the art having the benefit ofthis disclosure.

The present invention will now be described with reference to theattached figures. The words and phrases used herein should be understoodand interpreted to have a meaning consistent with the understanding ofthose words and phrases by those skilled in the relevant art. No specialdefinition of a term or phrase, i.e., a definition that is differentfrom the ordinary and customary meaning as understood by those skilledin the art, is intended to be implied by consistent usage of the term orphrase herein. To the extent that a term or phrase is intended to have aspecial meaning, i.e., a meaning other than that understood by skilledartisans, such a special definition will be expressly set forth in thespecification in a definitional manner that directly and unequivocallyprovides the special definition for the term or phrase.

In the specification, reference may be made to the direction of fluidflow between various components as the devices are depicted in theattached drawings. However, as will be recognized by those skilled inthe art after a complete reading of the present application, the deviceand systems described herein may be positioned in any desiredorientation. Thus, the reference to the direction of fluid flow shouldbe understood to represent a relative direction of flow and not anabsolute direction of flow. Similarly, the use of terms such as “above,”“below,” or other like terms to describe a spatial relationship betweenvarious components should be understood to describe a relativerelationship between the components as the device described herein maybe oriented in any desired direction.

Referring to FIG. 1, one illustrative embodiment of a completion systemaccording to the present invention includes a subsea Christmas tree 100comprising production valves 102 a-102 c and annulus valves 104 a-104 c.While FIG. 1 illustrates a particular number of production valves 102and annulus valves 104, the present invention is not so limited. Rather,the number of production valves 102 and annulus valves 104 will beimplementation specific. An upper end of the Christmas tree 100comprises a hub profile, indicated generally at 106, for connected to asubsea riser or other device. A subsea hydraulic connector 108 isdisposed on the lower end of the Christmas tree 100 for connecting to asubsea wellhead 402 (shown in FIG. 4). The Christmas tree 100 alsodefines a bore 112 at a lower end thereof. In the illustratedembodiment, the bore 112 comprises one or more internal sealing surfaces114 and an internal groove profile 116.

Referring now to FIG. 2, an illustrative embodiment of a tubing hangerassembly 200 according to the present invention comprises an annulartubing hanger body 202 having an upper end 204 and a lower end 206. Thelower end 206 of the tubing hanger body 202 is adapted to suspend atubing string 208 via a threaded connection 210 or by other suitablemeans. Note that the term “tubing string” is used herein to mean aproduction tubing string or any other type of tubing string. The upperend 204 of the tubing hanger body 202 is adapted for insertion into thebore 112 (shown in FIG. 1) of the Christmas tree 100. One or more sealsor seal assemblies are provided on the upper end of the tubing hangerbody 202 for sealing between the tubing hanger body 202 and the sealingsurfaces 114 of the Christmas tree bore 112. The seals or sealassemblies may comprise any of several suitable seal configurations thatare well known to the art. For example, the seals or seal assemblies maycomprise primary metal-to-metal seals 212 a, 212 b (known in the art as“SBMS seals”) and secondary elastomeric “S” seals 214 a, 214 b.

The tubing hanger assembly 200 further comprises an expandable lock ring216 disposed below a radially extending flange 218 of the tubing hangerbody 202. In the illustrated embodiment, a lock mandrel 220 is slidablydisposed about the tubing hanger body 202 below the lock ring 216. Thelock mandrel 220 rests upon an anti-backoff member 222, such as athreaded nut, which is threadedly engaged with the outer surface of thetubing hanger body 202 below the lock mandrel 220.

Referring also to FIG. 3, when the upper end 204 of the tubing hangerassembly 200 is inserted into the bore 112 of the Christmas tree 100,the seals (e.g., the metal-to-metal seals 212 a, 212 b and theelastomeric seals 214 a, 214 b) engage the sealing surfaces 114 of thebore 112. The Christmas tree 100 is thus sealed to the tubing hangerassembly 200. The lock mandrel 220 is then moved upward to a lockedposition using a hydraulic jack (not shown) or any other appropriatemeans. As the lock mandrel 220 moves toward its locked position, a firstend 224 thereof engages and expands the lock ring 216 outwardly in amanner known to the art. One or more surfaces 226 a, 226 b of the lockring 216 engage the profile 116 of the bore 112 of the Christmas tree100, thus locking the tubing hanger assembly 200 to the Christmas tree100. The anti-backoff member 222 is then advanced upwardly until itcontacts the lock mandrel 220, thus preventing the mandrel from movingaway from its locked position.

In the illustrated embodiment, the weight of the tubing string 208 andthe tubing hanger assembly 200 is transmitted from the external flange218 of the tubing hanger body 202, through the lock ring 216 and theChristmas tree profile 116, to the Christmas tree 100. In this way, thetubing hanger assembly 200 can be installed in the Christmas tree 100and the combined unit can be run into the well.

Note that any other suitable means can be employed within the scope ofthe present invention for connecting the tubing string 208 to theChristmas tree 100. For example, a hydraulically actuated connector, athreaded connector (e.g., a tubing thread or a low-turn casingconnector), a threaded retainer nut, or a shrink-fit coupling can beused to connect the tubing string 208 to the Christmas tree 100. In oneparticular alternative embodiment, shown in FIGS. 4A-4B, the tubingstring 208 is connected to the Christmas tree 100 via a union nutarrangement. A first union nut 400 retains a tubing hanger 402 to anintermediate spoolpiece 404. A second union nut 406 retains theintermediate spoolpiece 404 to the Christmas tree 100. A seal sleeve408, comprising seal elements 410, 412 on an upper end thereof and sealelements 414, 416 on a lower end thereof, serves as a conduit and sealbetween the tubing hanger 402 and the Christmas tree 100. Upon make upof the tubing hanger 402 into the intermediate spoolpiece 404, automaticcouplers 418 connect the downhole hydraulic and electrical circuits.

In another alternative embodiment, shown in FIG. 5, the tubing string208 is connected to the Christmas tree via a shrink fit coupling 500.The coupling 500 comprises a pin 502 attached to the tubing string 208generally at 504 and a box 506 coupled to the Christmas tree 100,generally at 508. The scope of the present invention, however,encompasses an intermediate coupling to attach the box 506 to theChristmas tree 100. Moreover, the pin 502 may be attached to theChristmas tree 100 and the box 506 attached to the tubing string 208.

An enlarged, cross-sectional view of a portion of the coupling 500 isshown in FIG. 6. The pin 502 and the box 506 each define complementarygrooves 602, 604 that engage one another. The box 506 also defines aninlet 606, through which a pressurized fluid (e.g., a gas or a liquid)may be introduced via a fluid flow path to the interface between the box506 and the pin 502. When such pressure is introduced through the inlet606, the box 506 is expanded and the pin 502 is compressed, such thatthe grooves 602, 604 are disengaged from one another, allowing the pin502 and the box 506 to be mated. When pressure is released from theinlet 606, the grooves 602, 604 engage one another to retain the pin 502and the box 506 together.

Still referring to FIG. 6, the coupling 500 may include, in certainembodiments, one or more sealing elements for inhibiting or preventingfluid from flowing between the pin 502 and the box 506. In theillustrated embodiment, the coupling 500 includes first and secondmetal-to-metal seals 608, 610. The seal 608 comprises a protrusion 612defined by the pin 502 that is received into and seals against a recess614 defined by the box 506. The seal 610 comprises a protrusion 616defined by the box 506 that is received into and seals against a recess618 defined by the pin 502. The coupling 500 also comprises a firstelastomeric seal 620 disposed in a recess 622 defined by the box 506that seals against a sealing surface 624 of the pin 502. Moreover, thecoupling 500 includes a second elastomeric seal 626 disposed in a recess628 defined by the pin 502 that seals against a sealing surface 630 ofthe box 506. Note that the present invention may include one or more ofthe seals 608, 610, 620, 626 in any chosen combination and location inthe coupling 500.

Referring to FIG. 7, when the Christmas tree 100, tubing hanger 200, andtubing string 208 are run as an assembly to the subsea wellhead 702, thesubsea hydraulic connector 108 of the Christmas tree 100 connects thelower end of the Christmas tree 100 to the wellhead hub profile 704 in amanner well known to the art. Note that any other suitable means can beemployed within the scope of the present invention to connect theChristmas tree 100 to the wellhead 702.

An illustrative installation sequence for the completion system detailedabove in relation to FIG. 1-FIG. 7 is described below, with reference toFIG. 8A-FIG. 8K. Note that the present invention is not limited to theinstallation sequence disclosed below. Referring now to FIG. 8A, asegmented, split stabbing guide 802 is installed over the subseawellhead 702 using a remotely operated vehicle 804 (shown in FIG. 8I).The stabbing guide 802 helps guide the end of the tubing string 208 intothe wellhead 702. As shown in FIG. 8B, the tubing string 208 issuspended from a rig or vessel (not shown) at the water surface. Thetubing hanger assembly 200 is installed onto the uppermost joint of thetubing string 208 and is supported using a tubing hanger running tool806. In the illustrated embodiment, a lower end of the tubing hangerrunning tool 806 defines a bore 808 that engages the tubing hangerassembly 200 in a manner similar to the engagement of the tubing hangerassembly 200 and the Christmas tree 100.

Referring now to FIG. 8C, the tubing hanger assembly 200 is then loweredto a split support structure 810 at a lower level of the rig or vessel(not shown), such as a moonpool of a rig. Note that the split supportstructure 810 can be moved between a non-engaged position and an engagedposition (shown in FIG. 8C) to support the tubing hanger assembly 200.The tubing hanger running tool 806 is then removed from the tubinghanger assembly 200.

Turning now to FIG. 8D, a set of adjustable or collapsible tree supports812 (illustrated in a stylized fashion in FIG. 8D) are then installed onthe support structure 810. The tree supports 812 can include mechanicalor hydraulic jacks or any other suitable device to allow elevationalpositioning of the Christmas tree 100 and/or the tubing string 208 (withthe tubing hanger assembly 200 installed thereon) with respect to oneanother. A lower riser package 814 is mated with the upper end 106 ofthe Christmas tree 100 and the Christmas tree 100/lower riser package814 assembly is landed on the tree supports 812 using a handling tool816. As illustrated in FIG. 8E, the tree supports 812 are thenmanipulated to either lower the Christmas tree 100 onto the tubinghanger assembly 200 or to raise the tubing hanger assembly 200 into theChristmas tree 100. The tubing hanger assembly 200 is then locked to theChristmas tree 100 in the manner described above in relation to FIG. 3.

Referring to FIG. 8F, a stress joint 818 and a riser 820 are attached tothe lower riser package 814 in place of the handling tool 816. TheChristmas tree 100 is then lifted from the tree supports 812 and thetree supports 812 are removed. As shown in FIG. 8G, the lower riserpackage 814, the Christmas tree 100, the tubing hanger assembly 200, andthe tubing string 208 are then run through open water to the subseawellhead 702 simultaneously (i.e., as a single unit), without the use ofa blow-out preventer. In certain configurations, a blow-out preventer orlike device may be used in conjunction with the present invention. Forexample, in certain embodiments, a shut off device or an environmentalsafety guard that is split may be employed, such that it may be removedafter landing the Christmas tree 100.

Referring now to FIG. 8H, a landing joint 822 and a surface flow tree824 are assembled at the rig or vessel (not shown), and a workoverumbilical 826 is prepared for installation of the Christmas tree 100. Asshown in FIG. 8I, the segmented stabbing guide 802 is released from thesubsea wellhead using the remotely operated vehicle 804. Then, asillustrated in FIG. 8J, the Christmas tree 100 is landed and locked tothe wellhead 702, after which the interface between the Christmas tree100 and the wellhead 702 is tested. Once testing is completed, thestress joint 818, the riser 820, and the lower riser package 814 areretrieved to the surface and a tree cap 828 is installed on theChristmas tree 100, as shown in FIG. 8K.

The present invention embodies a number of distinct advantages andbenefits. By making-up the tubing string 208 to the Christmas tree 100prior to installation in the wellhead 702, the critical connectionbetween the tubing string 208 and the Christmas tree 100 is simplified,thus reducing the cost of the connection and the number of potentialleak paths. Furthermore, this connection can be checked at the surface.Similarly, downhole hydraulic and electrical service and control linescan be connected and checked at the surface, thereby eliminating theuncertainty inherent in downhole connections made subsea. The hydraulicand electrical lines may be terminated and connected at the bottom ofthe Christmas tree 100, or they may be routed through the Christmas tree100, thus eliminating all connections below and within the Christmastree 100, which increases the reliability of these lines.

Attaching the tubing string 208 directly to the tree allows the size ofthe tubing and the production bore to be increased. Heavier tubingstrings 208 can be utilized without the use of exotic materials andwithout increasing the size of the wellhead 702. Thus, higher productionflow rates can be achieved. The Christmas tree 100 of the presentinvention includes all of the valves normally required for a vertical,or conventional, subsea completion system. However, because there are noplugs or barriers that must be pulled via the annulus passageway,wireline access to the annulus is not required. Thus, the annulus valves104 a-104 c may be moved to the side and/or placed in a separate wingblock, as illustrated in FIG. 1. This results in a significant reductionin height of the Christmas tree 100 as compared to a conventionalvertical Christmas tree. The need for wireline bore selection devices isalso eliminated.

The completion system of the present invention may be installed ineither an oriented or an un-oriented configuration. Aspects such oforiented and un-oriented configurations are presented in acommonly-assigned U.S. patent application entitled “Subsea CompletionSystem, and Methods of Using Same” by inventor Christopher D. Bartlett,filed on Oct. 14, 2004 and having attorney docket number 2065.003200,which is hereby incorporated by reference for all purposes. The systemmay be installed without the use of guide funnels because the tubingstring 208 itself guides the Christmas tree 100 into position uponlanding on the wellhead 702. Another cost-saving feature of the presentinvention is the elimination of all remote installation and retrievaltools for the tubing hanger.

Alternatively, as shown in FIG. 9A-FIG. 9D, the tubing hanger assembly200 and tubing string 208 can be run to the wellhead 702 independentlyof the Christmas tree 100. As shown in FIG. 9A, the tubing hangerassembly 200 and tubing string 208 is also run through open-water viathe tubing hanger running tool 806 and without a blow-out preventer. Insuch an embodiment, the tubing hanger assembly 200 would be supported inthe wellhead 702, as illustrated in FIG. 9B, and, in someconfigurations, latched thereto. After removing the tubing hangerrunning tool 806 (shown in FIG. 9C), the Christmas tree 100 is then runsubsea and latched to the wellhead 702, the tubing hanger assembly 200,or both (shown in FIG. 9D). It is contemplated that the means forlatching the tubing hanger assembly 200 to the Christmas tree 100 couldbe operable independently of the means for latching the tubing hangerassembly 200 to the wellhead 702. Thus, the tubing hanger assembly 200could be latched to the Christmas tree 100, the wellhead 702, or both,simultaneously.

It is also contemplated that a single means could be employed that wouldlatch the tubing hanger assembly 200 to either the wellhead 702 or theChristmas tree 100 and simultaneously unlatch the tubing hanger assembly200 from the other of these two components. In this way, the Christmastree 100 could be retrieved at a later point in time withoutnecessitating the retrieval of the entire completion string (ie., theentire completion system). Alternatively, if it is desired to alsoretrieve the tubing string 208, then the entire completion string can beretrieved in combination with the retrieval of the subsea Christmas tree100.

In another illustrative embodiment, shown in FIG. 10, a tubing hanger1002 is landed in and latched to the wellhead 702 and the latching means1000 is housed in the wellhead 702. In such an embodiment, the outsidediameter of the tubing hanger 1002 includes only locking grooves 1004and is reduced in size. The bore 1005 of the Christmas tree 1006 is thenof sufficient size to drift the tubing hanger 1002, so that the tubingstring 208 and the tubing hanger 1002 can be landed through theChristmas tree 1006. Alternatively, the tubing string 208 and the tubinghanger 1002 can be landed before the Christmas tree 1006.

In yet another alternative embodiment, shown in FIG. 11, the tubinghanger assembly 200 is landed and locked in a separate tubing head 1102.The Christmas tree 100 is then attached to the top of the tubing head1102 at the surface. The tubing head 1102, tubing hanger assembly 200,and Christmas tree 100 are all run through open-water to the wellhead702 simultaneously.

An embodiment is also contemplated, as shown in FIG. 12, wherein thetubing hanger 1202 is landed in and latched to a tubing head 1204 andthe latching means 1206 is housed in the tubing head 1204. In such anembodiment, the outside diameter of the tubing hanger 1202 includes onlylocking grooves 1208 and is reduced in size.

In one embodiment, a subsea completion system according to the presentinvention includes a tubing string, a Christmas tree defining a bottomopening adapted to have the tubing string inserted therein, and meansfor latching the tubing string to prevent movement relative to theChristmas tree. A method of using the subsea completion system includescoupling a tubing string with a Christmas tree above water and landingthe Christmas tree on a subsea wellhead. A subsea Christmas tree definesa lower bore adapted to mate with a tubing string, such that the tubingstring is latchable in the Christmas tree.

This concludes the detailed description. The particular embodimentsdisclosed above are illustrative only, as the invention may be modifiedand practiced in different but equivalent manners apparent to thoseskilled in the art having the benefit of the teachings herein.Furthermore, no limitations are intended to the details of constructionor design herein shown, other than as described in the claims below. Itis therefore evident that the particular embodiments disclosed above maybe altered or modified and all such variations are considered within thescope and spirit of the invention. Accordingly, the protection soughtherein is as set forth in the claims below.

1. A subsea completion system, comprising: a tubing string; a Christmastree defining a bottom opening adapted to have the tubing stringinserted therein; and means for latching the tubing string to preventmovement relative to the Christmas tree.
 2. A subsea completion system,according to claim 1, wherein the means for latching the tubing stringin the subsea completion system comprises a tubing hanger assemblyadapted to be operatively coupled with the Christmas tree.
 3. A subseacompletion system, according to claim 2, wherein the Christmas treedefines a bore including a sealing surface and a groove profile and thetubing hanger assembly comprises: a body including an upper end receivedin the bore of the Christmas tree and a lower end threadedly connectedto the tubing string; a seal disposed on the upper end engaged with thesealing surface of the Christmas tree bore for sealing between theChristmas tree and the body; a lock ring disposed about the body; ananti-backoff member threadedly engaged with the body; and a lock mandrelslidably disposed about the body and resting on the anti-backoff member,the lock mandrel being engageable with the lock ring to expand the lockring and engage the groove profile of the Christmas tree.
 4. A subseacompletion system, according to claim 3, wherein the anti-backoff membercomprises an anti-backoff nut.
 5. A subsea completion system, accordingto claim 3, wherein the Christmas tree bore includes a first sealingsurface and a second sealing surface and the tubing hanger assemblycomprises a metal-to-metal seal engaged with the first sealing surfaceand an elastomeric seal engaged with the second sealing surface.
 6. Asubsea completion system, according to claim 1, wherein the Christmastree comprises an annulus valve disposed on the side of the Christmastree or in a separate wing block of the Christmas tree.
 7. A subseacompletion system, according to claim 1, wherein the tubing string islatched above water.
 8. A subsea completion system, according to claim1, further comprising a tubing hanger, wherein the tubing string islatched to the Christmas tree via the tubing hanger.
 9. A subseacompletion system, according to claim 1, further comprising a tubinghanger and a tubing head, wherein the tubing string is latched to theChristmas tree via the tubing hanger and the tubing head.
 10. A subseacompletion system, according to claim 9, wherein the tubing hangercomprises a latch.
 11. A subsea completion system, according to claim 9,wherein the tubing head comprises a latch.
 12. A subsea completionsystem, according to claim 1, further comprising a tubing hanger,wherein the tubing string is adapted to be latched to a well head viathe tubing hanger.
 13. A subsea completion system, according to claim 1,wherein the means for latching comprises one of a hydraulically actuatedconnector, a threaded connector, a threaded retainer nut, and ashrink-fit coupling.
 14. A subsea completion system, according to claim13, wherein the shrink-fit coupling comprises: a pin attached to one ofthe tubing string and the Christmas tree; and a box defining a fluidflow path and attached to the other of the tubing string and theChristmas tree, the box being operatively engageable with the pin,wherein the box is disengaged from the pin when a pressurized fluid istransmitted through the fluid flow path and the box is engaged with thepin when a pressurized fluid is absent from the fluid flow path.
 15. Asubsea completion system, according to claim 14, further comprising asealing element for inhibiting or preventing fluid flow between the boxand the pin.
 16. A system adapted to be coupled to a subsea wellhead,comprising: a tubing hanger adapted to be positioned in the wellhead,the tubing hanger defining a flow opening extending therethrough; and aChristmas tree operatively coupled to the tubing hanger above water. 17.A system, according to claim 16, further comprising a tubing stringattached to the tubing hanger and defining a flow opening therethrough,such that the flow opening of the tubing string and the flow opening ofthe tubing hanger are in fluid communication.
 18. A system, according toclaim 16, wherein the Christmas tree is operatively coupled to thetubing hanger via a tubing head.
 19. A method, comprising: coupling atubing string with a Christmas tree above water; and landing theChristmas tree on a subsea wellhead.
 20. A method, according to claim19, wherein coupling the tubing string with the Christmas tree comprisesinstalling a tubing hanger on an uppermost joint of the tubing stringand locking the tubing hanger to the Christmas tree.
 21. A method,according to claim 20, wherein locking the tubing hanger to theChristmas tree comprises expanding a lock ring of the tubing hanger. 22.A method, according to claim 21, wherein expanding the lock ring of thetubing hanger comprises engaging a lock mandrel of the tubing hangerwith the lock ring.
 23. A method, according to claim 20, furthercomprising lowering the tubing hanger, with the tubing string attached,to a support structure of a rig or vessel.
 24. A method, according toclaim 23, further comprising landing the Christmas tree on tree supportsdisposed on the support structure.
 25. A method, according to claim 24,further comprising manipulating the tree supports to mate the Christmastree and the tubing hanger.
 26. A method, according to claim 19, furthercomprising guiding the Christmas tree to the subsea wellhead using thetubing string.
 27. A method, according to claim 26, further comprisinginstalling a stabbing guide over the subsea wellhead before guiding theChristmas tree to the subsea wellhead.
 28. A method, according to claim27, wherein installing the stabbing guide comprises installing thestabbing guide with a remotely operated vehicle.
 29. A method, accordingto claim 19, further comprising running the Christmas tree, the tubinghanger, and the tubing string open-water to a well extending from thesubsea wellhead.
 30. A method, according to claim 29, wherein runningthe Christmas tree, the tubing hanger, and the tubing string furthercomprises running the Christmas tree, the tubing hanger, and the tubingstring without a blow-out preventer.
 31. A method, according to claim19, further comprising removing a stabbing guide from the subseawellhead before landing the Christmas tree on the subsea wellhead.
 32. Amethod, according to claim 31, wherein removing the stabbing guidecomprises removing the stabbing guide with a remotely operated vehicle.33. A method, according to claim 19, wherein coupling the tubing stringwith the Christmas tree comprises installing a tubing hanger on anuppermost joint of the tubing string, locking the tubing hanger in atubing head, and attaching the tubing head to the Christmas tree.
 34. Amethod, comprising: coupling a tubing string with a tubing hanger abovewater; landing the tubing hanger on a subsea wellhead; and landing aChristmas tree on the subsea wellhead.
 35. A method, according to claim34, further comprising latching the tubing hanger to the Christmas tree.36. A method, according to claim 34, further comprising latching thetubing hanger to the wellhead.
 37. A method, according to claim 34,wherein landing the tubing hanger on the subsea wellhead furthercomprises landing the tubing hanger on the subsea wellhead via a tubinghead and latching the tubing hanger to the tubing head.
 38. A subseaChristmas tree defining a lower bore adapted to mate with a tubingstring, such that the tubing string is latchable in the Christmas tree.39. A subsea Christmas tree, according to claim 38, wherein the lowerbore is adapted to mate with the tubing string via one of a tubinghanger, a hydraulically actuated connector, a threaded connector, athreaded retainer nut, and a shrink fit coupling.